1. Field of the Invention
This invention relates in general to wireless communication systems and in particular to the communication of calls within a wireless communication system.
2. Description of the Related Art
Wireless communication systems today employ a variety of modulation schemes from analog modulation to digital modulation such as binary phase modulation and binary frequency modulation. The type of modulation utilized greatly affects the performance of the wireless communication system. Considerable effort is spent in the optimization of key performance elements such as bandwidth, demodulator performance and error control strategy for overall enhancement of data throughput.
As the available spectrum for wireless communications becomes a premium due to the increasing number of subscribers, more bandwidth efficient transmission methods are required. Recently, designers have demonstrated that multilevel modulation schemes such as multilevel phase shift keying and quadrature amplitude modulation (QAM) provide efficient modulation techniques to meet the performance requirements of the wireless communication systems. Of these, QAM, a bandwidth efficient transmission method, yields the highest potential data throughput by its use of amplitude as well as phase modulation.
QAM is the amplitude modulation and demodulation of two carriers that have the same frequency but are in phase quadrature to each other. It can be either analog or digital. QAM has been shown to be a suitable modulation scheme for wireless communication systems.
Further, variable rate QAM modulation has been found to be an efficient modulation scheme capable of substantial performance gains over conventional QAM. In variable rate QAM modulation, the wireless communication device and the base station transmitter coordinate to determine an optimum modulation rate based on signal conditions and channel loading. The baud rate in such a system is fixed, but multiple modulation levels are allowed (2 QAM, 4 QAM, 16 QAM etc.). QAM transmissions over wireless communication channels are subject to a variety of impairments, including interference, noise, multipath fading, and delay spread. In general, higher modulation levels (16 QAM, 64 QAM, etc.) are less tolerant to these impairments than lower modulation levels (2 QAM or 4 QAM). With variable rate QAM techniques, the modulation level is adapted to the existing channel conditions, usually using the highest modulation level which meets some minimum performance requirements (Bit error rate, packet loss, etc.)
As a further enhancement to conventional variable QAM modulation, a pilot carrier can be transmitted along with the data to provide a reference for coherent demodulation. For a further description of this enhancement, please refer to U.S. Pat. No. 4,816,783 by Leitch, entitled xe2x80x9cMethod and Apparatus for Quadrature Modulationxe2x80x9d, and assigned to Motorola Inc. of Schaumburg, Ill., the assignee of the present invention.
The improved system capacity achieved must be balanced with the increased cost of the wireless communication system due to the increased complexity of variable rate QAM modulation techniques. What is needed is a method to reduce the complexity of the implementation of variable rate QAM modulation between the base station transmitters and the wireless communication devices in a wireless communication system.